(1) The structure and sequence of the human c-sis/PDGF-2 transcriptional unit has been determined. The role of various sequences in the gene locus, including the 5' and 3' flanking sequences, in the regulation of tissue-specific expression of this prototype growth factor with transforming potential was investigated in endothelial cells and fibroblasts that do and do not express the c-sis/PDGF-2 transcript, respectively. By utilizing the bacterial chloramphenicol acetyl transferase gene as reporter, we functionally localized the c-sis/PDGF-2 promoter 23 bp upstream of the mRNA cap site. Within a 4-kbp region upstream of the mRNA cap site, there were no sequences that conferred tissue-specific differences in reporter gene expression. Inhibiting sequences were detected upstream of the promoter but lacked cell specificity. The lack of tissue specificity of the c-sis/PDGF- 2 gene promoter is further established by nuclear run-on analysis which demonstrated constitutive transcriptional activity of the endogenous c-sis/PDGF-2 promoter in fibroblasts. All of these findings imply that c-sis/PDGF-2 RNA expression is normally regulated at a post-transcriptional rather than transcriptional level. (2) The 5' untranslated sequence (5' UTS) of the c-sis mRNA was shown to exert a potent inhibitory effect on translation. Deletion of 5' UTS resulted in as much as a 40-fold increase in translation, independent of the reporter gene or cell type analyzed. A DNA construct containing the c-sis/PDGF-2 transcriptional unit lacked detectable biological activity upon transfection of NIH/3T3 fibroblasts, but deletion of the 5' UTS unmasked c-sis/PDGF-2 transforming activity. Thus, the normal mechanisms which inhibit transforming activity of the c- sis/PDGF-2 proto-oncogene in fibroblasts are exerted at post- transcriptional levels.